This invention relates to RF signal transmission systems and, more particularly, to RF signal transmission systems where the signal source is remote from the load.
There are many applications where a phase-stable RF signal must be transported some distance from the signal source to a load. For example, in the Ground Test Accelerator (GTA) signals must be transported between a RF master reference generation oscillator and each individual control rack (RF Reference Transport) as well as between RF cavities and the associated control racks located away from the cavities (Field Sense Transport), where phase errors contribute directly to errors in cavity-to-cavity phase of the accelerator. The GTA system requires an insertion phase tolerance of .+-.0.15 degrees for each transport cable. This system is a fixed frequency system and frequency-dependent effects are not of concern. Temperature changes, however, affect the electrical length and transmission delay time of the coaxial cable, with concomitant changes in the signal insertion phase. For example, over a 250-foot run of cable, an ambient temperature range of 23.degree. to 43.degree. C. can introduce as much as 8 degrees of electrical phase change at 425 MHz in phase stabilized coaxial cable, more than 50 times the tolerable limit for GTA.
There have been several attempts to solve this problem. In one approach, a controlled environment is maintained for the transmission cable. In another approach, described in H. D. Schwarz et al., "The RF Reference Line for PEP," NS-26 IEEE Trans. Nucl. Sci., pp. 1987-1989 (March 1989), a portion of the RF signal is modulated at the accelerator end of the cable and reflected back to the RF signal end of the cable. The signal is demodulated and the phase change in the modulation signal is determined for use in shifting the RF signal phase at the accelerator. However, active components are required adjacent the accelerator and they are not suitable for use in a radiation environment. Yet another approach used a fiber optic RF distribution system, but still required active components in close proximity to the RF cavities.
In accordance with the present invention, a phase-stabilized RF signal is delivered to a load that is remote from the signal source where the phase stabilization is obtained with only passive components adjacent the load. Accordingly, it is an object of the present invention to provide a phase control loop with only passive components adjacent the load.
It is another object of the present invention to provide a phase control loop for use in a high radiation environment.
One other object of the present invention is to obtain a phase-stabilized RF signal path without the need for precise environmental control.
Yet another object of the present invention is to maintain the phase-stabilized RF signal path in the absence of the RF signal to enable the phase-stabilized transport of pulsed RF signals.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.